A ninny is a dull-witted person (in North America they're sometimes called ninnyhammers).
How easy a spot is this!
Obviously most politicians of all the other parties (and some of yours) will qualify, as will the person between you and the shelf at the supermarket; the person blocking the road junction; almost all the journalists on several newspapers easily available in your local area; at least three of the people in your class/office/family (quite possibly including a boss/teacher/parent); the interviewee on the radio; the presenter on the radio; the neighbour who double-parks; the person who displays his musical taste for everyone to hear; and, of course, the dog who eats wallets and the white cat who sleeps on the black velvet jacket.
Mind you, they say it takes one to know one.
As proof of this, see below, because the word ninny seems to have been made up by a real idiot...
Spot the Frippet: ninny. This word seems to be an example of false splitting based on the phrase an innocent (from when, in the 1500s, innocent meant simpleton), This became, in the mouths of the ignorant, a ninnocent and was shortened from there.
Where the hammer in ninnyhammer came from no one knows, but it might be just to rhyme with yellowhammer. (The hammer in yellowhammer probably comes from a German word for the sort of bird English speakers call a bunting.)